Abstract

Our recent studies show that chronic oral nicotine partially protects against striatal damage in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated nonhuman primates. To identify the cellular changes associated with this protective action, we investigated the effects of nicotine treatment on stimulus-evoked dopamine release, dopamine turnover, and synaptic plasticity in striatum from lesioned and unlesioned animals. Monkeys were chronically (6 months) treated with nicotine in the drinking water and subsequently lesioned with the dopaminergic neurotoxin MPTP (6 months) while nicotine was continued. Nigrostriatal damage increased nicotinic acetylcholine receptor (nAChR)-mediated fractional dopamine release from residual terminals, primarily through changes in α3*/α6* nAChRs. In contrast, fractional receptor-evoked dopamine release was similar to control in unlesioned and lesioned animals with chronic oral nicotine. Long-term nicotine administration also attenuated the enhanced K+-evoked fractional dopamine release from synaptosomes of MPTP-lesioned animals, suggesting that nicotine treatment had a generalized effect on dopaminergic function. This premise was further supported by experiments showing that nicotine dosing decreased the elevated dopamine turnover that occurs after nigrostriatal damage. We next investigated changes in synaptic plasticity with lesioning and nicotine treatment. Nicotine treatment alone enhanced synaptic plasticity by lowering the threshold for long-term depression (LTD) in the corticostriatal pathway. MPTP lesioning led to a loss of LTD, a measure of short-term synaptic plasticity. In contrast, LTD was preserved in nicotine-treated lesioned animals. Thus, the present data show that the disruptions in striatal dopaminergic function after nigrostriatal damage were attenuated with chronic nicotine administration. These cellular alterations may underlie the ability of nicotine to maintain/restore normal function with nigrostriatal damage.

To further elucidate the mechanism involved in this phenomenon, we investigated the effect of nicotine treatment on long-term depression (LTD) in corticostriatal slices from unlesioned and MPTP-lesioned primates. Previous work had shown that synaptic plasticity, as measured using LTD, is severely impaired after dopaminergic denervation in rodents (Calabresi et al., 1992a,b; Centonze et al., 1999, 2001). In the present study, we tested whether nicotine administration prevented this loss of plasticity in corticostriatal slices from MPTP-lesioned primates.

Materials and Methods

Animals.

Adult female squirrel monkeys (Saimiri sciureus) were purchased either from Osage Research Primates (Osage Beach, MO) or the University of South Alabama (Mobile, AL). Animals were housed separately in a room with a 13/11 h light/dark cycle and were fed once daily with ad libitum access to water. A standard 4 week quarantine period was initiated on arrival. Before treatment, the monkeys were allowed to acclimatize to the home environment for ≥1 month period. Animals were then randomly assigned to one of four treatment groups: control (n = 7), nicotine-treated (n = 6), MPTP-treated (n = 7), or nicotine plus MPTP-treated (n = 6). All monkeys were given saccharin (1% solution) in the drinking water for a 2 week period. Animals in the nicotine treatment group were subsequently given ascending doses of nicotine in the drinking water, beginning with 25 μg/ml for 1 week, 50 μg/ml for 1 week, and increasing by 50 μg increments/week over ∼3–4 months up to a final concentration of 650 μg/ml (Quik et al., 2005b). The nicotine/saccharin solution was also added to food pellets that were given once daily. After 6 months of nicotine treatment, monkeys were given three doses of MPTP (1.5 mg/kg, s.c.) over a 6 month period at 2 month intervals with nicotine treatment continued until 24 h before death. The animals were killed according to the recommendations of the Panel on Euthanasia of the American Veterinary Medical Association. They were first injected intraperitoneally with euthanasia solution (390 mg/ml sodium pentobarbital and 50 mg/ml phenytoin sodium), followed by 2.2 ml/kg (i.v.) of the same solution. All procedures conform to the National Institutes of Health Guide for the Care and Use of Laboratory Animals and were approved by the Institutional Animal Care and Use Committee.

Tissue preparation.

The brain was rapidly removed and sectioned along the midline, and tissue was prepared as follows. One-half was placed in a squirrel monkey Plexiglas brain mold and sliced into 6-mm-thick sections, which were placed on glass slides and quickly frozen in isopentane on dry ice for autoradiographic studies. The mesencephalon containing the substantia nigra was dissected from the other one-half and fixed for tyrosine hydroxylase cell counts, while the remainder was sliced into 2 mm sections using the brain block. The caudate and putamen were dissected from the anterior slices (A15–A13) for measurement of dopamine and metabolite levels, dopamine uptake, and dopamine release, while the slice at level A13–A12 was used for electrophysiological recording.

Dopamine transporter autoradiography.

Binding of (3β-(4–9125I)iodophenyl)tropane-2β-carboxylic acid isopropyl ester (125I-RTI-121) (2200 Ci/mmol; PerkinElmer, Wellesley, MA) was done to identify the dopamine transporter. Striatal sections were preincubated for 15 min two times in 50 mm Tris buffer, pH 7.4, plus 120 mm NaCl and 5 mm KCl. This was followed by a 2 h incubation with 50 pm125I-RTI-121 in the same buffer also containing 0.025% bovine serum albumin and 1 μm fluoxetine. To terminate binding, sections were washed for 15 min four times in ice-cold buffer and once in ice-cold deionized H2O for 10 s. Slides were air-dried and exposed to Kodak MR film (Amersham Biosciences, Piscataway, NJ) for ∼3 d. Nonspecific binding, determined in the presence of nomifensine (100 μm), a dopamine uptake inhibitor, was similar to the film blank.

Dopamine and its metabolites.

Approximately 5 mg of tissue (each) was dissected from the medial and lateral caudate, and the dorsal and ventral putamen. Dopamine and its metabolites homovanillic acid (HVA) and dihydroxyphenylacetic acid (DOPAC) were extracted into 250 μl of 0.4N perchloric acid. Catecholamines were determined using high-pressure liquid chromatography coupled to electrochemical detection (Quik and Di Monte, 2001).

Electrophysiology.

After dissection, the 2 mm coronal monkey brain slice at level A13–A12 was bisected at the level of the internal capsule, and the portion containing the putamen was quickly placed in ice-cold, oxygenated buffer containing the following (in mm): 248 sucrose, 1.5 KCl, 6.0 MgSO4, 1.0 CaCl2, 1.25 NaH2PO4, 26 NaHCO3, and 11 glucose, pH 7.4. It was then cut into 300–350 μm slices, using a vibratome in a plane to include cortex and putamen such that the corticostriatal connections between these two regions was maintained (Chen et al., 2006). The slices were incubated at room temperature for ≥1 h in oxygenated artificial CSF (ACSF), pH 7.4, containing the following (in mm): 124 NaCl, 2.5 KCl, 1.5 MgSO4, 2.0 CaCl2, 1.25 NaH2PO4, 26 NaHCO3, and 11 glucose. A slice was transferred to the recording chamber and perfused with ACSF at 35°C. Extracellular striatal field potentials were then recorded from the dorsal putamen as described previously (Chen et al., 2006) by bipolar stimulation (0.1 Hz; 100 μs; 6–8 V) of the cerebral cortex or white matter between the cortex and putamen. Synaptic plasticity was determined by comparing average peak amplitude or slope of the striatal field potentials before and after high-frequency stimulation (HFS) (100 Hz; 100 μs; 14 V pulses). The quantitative data are expressed as a percentage of baseline, which represents the average amplitude of evoked field potentials, recorded after stabilization of the response 10–20 min before tetanic HFS.

Data analyses.

[3H]Dopamine release was quantitated as previously described (McCallum et al., 2005). Rmax and EC50 values were obtained from dose–response curves by fitting data to a nonlinear regression equation in SigmaPlot 2001 for Windows (SPSS, Chicago, IL). Fractional release was determined by dividing release for a particular sample by the dopamine transporter value to obtain a measure of release from residual dopaminergic terminals as reported previously (Snyder et al., 1990).

Statistical analyses were done using one- or two-way ANOVA followed by either Newman–Keuls or Bonferroni’s post hoc test, respectively, using GraphPad Prism (San Diego, CA). All values are expressed as the mean ± SEM of the indicated number of animals. A level of p < 0.05 was considered significant.

Results

Animal model

For these studies, we provided nicotine to the monkeys in the drinking water, as this approach has the advantage that administration is pulsatile because animals drink sporadically throughout the day and yet chronic because treatment is provided over many months. As previously reported, this nicotine-dosing regimen resulted in plasma nicotine levels of 12.6 ± 1.3 ng/ml (n = 12) and cotinine levels of 370 ± 47 ng/ml (n = 12), which are within the range of those in the plasma of smokers (Quik et al., 2005b). The chronic nicotine treatment was well tolerated; the animals appeared well on physical examination, and there were no significant differences in body weight or fluid intake among the groups (Quik et al., 2005b). The animals’ activity was also measured using a computerized activity monitor system (Togasaki et al., 2005) with no differences in total daily activity among the four treatment groups (Quik et al., 2005b).

Low-dose MPTP (1.5 mg/kg) was administered subcutaneously three times at 2 month intervals to generate a relatively slow prolonged lesion. We used dopamine transporter autoradiography (125I-RTI-121) as an index of dopaminergic damage, because this measure may reflect functional transporter sites at the terminal (Quik et al., 2001). MPTP treatment led to significant 40–80% decreases in dopamine transporter binding in the different striatal regions (Table 1). Two-way ANOVA yielded an overall significant main effect of MPTP lesioning (p < 0.001) in every region, consistent with previously reported measurements of striatal dopamine transporter using Western blots (Quik et al., 2005b). There was also a significant main effect of nicotine dosing (p < 0.05) in each striatal region, with no significant interaction between MPTP and nicotine treatment.

Experiments were next done to determine the effect of chronic nicotine in the drinking water on nAChR-evoked dopamine release from striatal synaptosomes of unlesioned and lesioned monkeys (Table 2). Because MPTP treatment resulted in significant nigrostriatal damage or loss of dopamine terminals as reflected by a decline in the dopamine transporter (Table 1), we determined nicotine-evoked fractional [3H]dopamine release from synaptosomes prepared from striatum of unlesioned and lesioned monkeys. To measure fractional release or release from residual terminals, maximal nAChR-evoked dopamine release (Rmax) was divided by dopamine transporter values as described previously (Snyder et al., 1990). The results in Figure 1 show that nicotine treatment had no significant effect on fractional nicotine-evoked [3H]dopamine release in unlesioned animals. In contrast, nigrostriatal damage significantly enhanced fractional release in all striatal subregions (p < 0.05). Chronic nicotine treatment significantly decreased the enhanced fractional release in MPTP-lesioned compared with unlesioned monkeys in both ventral and dorsal putamen (p < 0.05). There were also declines in the fractional release in the caudate with nicotine administration; however, these did not reach statistical significance.

nAChR-mediated dopamine release in the primate striatum is primarily mediated by α3*/α6* and α4* nAChR subtypes (McCallum et al., 2005), with the asterisks indicating the presence of other subunits in the receptor complex. To differentiate these two components of nicotine-evoked release, we used the selective α3*/α6* nAChR antagonist α-conotoxinMII to define α3*/α6* nAChR-evoked fractional release (α-conotoxinMII-sensitive) and α4* nAChR-mediated fractional release (α-conotoxinMII-resistant). The results in Figure 2 show that there was a small decline in α3*/α6* nAChR-mediated (or α-conotoxinMII-sensitive) fractional dopamine release with chronic nicotine treatment in some striatal regions as reported previously (McCallum et al., 2006b). Nigrostriatal damage significantly (p < 0.01) enhanced release in both caudate and putamen. In contrast, in lesioned monkeys treated with nicotine, a reduction was observed in striatal α-conotoxinMII-sensitive nicotine-evoked dopamine release compared with lesioned animals not treated with nicotine (p < 0.01).

α4* nAChR-mediated (α-conotoxinMII-resistant) [3H]dopamine release was also analyzed in the same treatment groups (Fig. 3). There was no change in fractional α4* receptor-evoked dopamine release in animals treated only with nicotine. A significant increase was obtained in both ventral and dorsal putamen with nigrostriatal damage. This enhanced response was reduced with nicotine treatment, although significantly (p < 0.05) only in the ventral putamen. This contrasts with the lack of change in α4* receptor-stimulated fractional release in the caudate under any condition.

Experiments were subsequently done to determine whether the effect of chronic nicotine treatment was selective for nicotine-evoked [3H]dopamine release or whether there were also compensatory changes in fractional K+-evoked release. The results in Figure 4 show that nicotine treatment alone had no effect on fractional K+-evoked [3H]dopamine release in unlesioned animals. In contrast, nigrostriatal damage significantly enhanced release in the ventral and dorsal putamen (p < 0.05), but not caudate. This enhanced release in putamen was no longer observed in MPTP-lesioned monkeys receiving chronic oral nicotine.

Numerous studies have shown that there are significant changes in dopamine and metabolite levels after nigrostriatal damage in Parkinson’s disease and in animal models (Zigmond et al., 1990; Hornykiewicz, 2001). To determine the effect of chronic nicotine treatment with nigrostriatal damage, dopamine, DOPAC, and HVA were determined in monkey striatum (Table 3). The results show that these measures were reduced with nigrostriatal damage in every region of striatum (p < 0.01). Nicotine treatment had no effect on dopamine levels in unlesioned animals, although there was a significant main effect of nicotine (p < 0.01) in lesioned animals as shown previously (Quik et al., 2005b).

The ratio of dopamine levels to those of the dopamine metabolites DOPAC and HVA (dopamine/DOPAC plus HVA), or dopamine turnover, was subsequently calculated (Fig. 5) as a measure of dopamine clearance by remaining dopaminergic neurons (Zigmond et al., 1990; Hornykiewicz, 2001). An increase was observed in dopamine turnover in all subregions of striatum from lesioned monkeys, with the greatest increases (p < 0.01) in the lateral caudate and dorsal putamen, that is, the regions with the greatest dopaminergic loss. Dopamine turnover rates in animals treated with nicotine and MPTP were significantly lower than in animals treated only with MPTP in medial caudate (p < 0.05), lateral caudate (p < 0.05), and ventral putamen (p < 0.01), although not dorsal putamen.

Corticostriatal slices for electrophysiological recording were prepared from brain sections at level A12–A13. Cortical stimulation-evoked field potentials (EFPs) were evoked by bipolar stimulation of projection fibers in ACSF containing Mg2+. Initial studies to establish control parameters for in vitro electrophysiological recording in monkey slices showed that the amplitude of EFPs was dependent on stimulus strength (our unpublished observations). Subsequent tetanus stimulation (100 Hz; 100 μs; 14 V; 20–900 pulses) of the corticostriatal pathway induced a LTD, which is the predominant change in plasticity observed using physiological concentrations of Mg2+ (Centonze et al., 2001; Pisani et al., 2005). Corticostriatal LTD recordings were obtained from the majority (7 of 10) of control monkeys, with representative data from one monkey depicted in Figure 6A. The LTD induction threshold was defined as the minimum number of HFS pulses necessary to generate a significant long-term (>20 min) decrease in EFP amplitude. In contrast, tetanus stimulation failed to induce LTD in slices from any MPTP-lesioned monkey (0 of 3) (Fig. 6B), consistent with studies in rodents (Calabresi et al., 1992a,b; Centonze et al., 1999, 2001; Chen et al., 2006). Stimulation of the corticostriatal pathway in slices from animals chronically treated with nicotine (Fig. 6C) also resulted in LTD in three of the four treated animals. In nicotine-treated MPTP-lesioned animals, a robust LTD was observed (Fig. 6D) in four of the five.

Effect of chronic nicotine treatment and nigrostriatal damage on the induction of corticostriatal LTD. Monkeys were chronically treated with nicotine in the drinking water and subsequently lesioned with MPTP. Corticostriatal slices were prepared from level A13–A12 as described in Materials and Methods. They were stimulated with a varying number (20–900) of HFS pulses (100 Hz; 100 μs; 14 V) and cortical stimulation EFPs were recorded. A, Evoked field potential amplitudes (expressed as a percentage of baseline) every 10 s in a slice preparation from an unlesioned monkey shows that HFS at 600 pulses induced LTD. B, Nicotine treatment appears to reduce the threshold for corticostriatal LTD induction to 60 pulses. C, LTD was not observed with HFS (up to 900 pulses) after nigrostriatal damage. D, However, LTD was observed in the MPTP-lesioned group with chronic nicotine treatment. The results in A–D are from a representative animal in each group.

The combined results from the monkeys in each group exhibiting LTD is depicted in Figure 7. The results show that chronic nicotine treatment significantly reduced the threshold for LTD induction in both unlesioned and MPTP-lesioned monkeys. These data suggest that chronic oral nicotine treatment enhances synaptic plasticity in both unlesioned and MPTP-lesioned monkeys.

Chronic nicotine treatment reduces the threshold for LTD induction in unlesioned and MPTP-lesioned monkeys. The threshold of LTD induction was defined as the minimal number of HFS pulses required to generate a significant long-term (>20 min) decrease in EFP amplitude compared with baseline. Shown are the combined results from three to four monkeys per group, with each bar representing the mean ± SEM of amplitude of EFPs after HFS (expressed as a percentage of baseline). Significance of difference from baseline was determined using one-way ANOVA followed by a Newman–Keuls post hoc multiple comparison test (*p < 0.05).

Discussion

The present experiments are the first to show that chronic oral nicotine normalizes aberrant striatal function that occurs as a consequence of nigrostriatal damage. To approach this, we evaluated several measures of striatal activity. These include dopamine release, dopamine turnover, and synaptic plasticity, with the latter defined by changes in LTD. We first determined nAChR-mediated dopamine release (Wonnacott, 1997), a measure directly linked to nicotine treatment. Chronic nicotine dosing normalized the lesion-induced increase in nicotine-evoked release from residual dopamine terminals in monkey striatum. Unexpectedly, nicotine treatment also reduced K+-evoked fractional dopamine release and dopamine turnover, which were both elevated as a result of nigrostriatal damage (Zigmond et al., 1984, 1990; Zigmond, 1997; Hornykiewicz, 1998, 2001; McCallum et al., 2006a). These combined data indicate that nicotine treatment not only restores functional measures directly associated with nAChR stimulation but also results in a generalized return of striatal function to normal after chronic nigrostriatal damage.

We also investigated the effect of nicotine treatment on striatal plasticity in both unlesioned and lesioned animals because changes in synaptic strength are implicated in motor function (Pisani et al., 2005). Repetitive stimulation of the corticostriatal pathway can induce long-term potentiation and LTD, with the direction of the change in plasticity dependent on the level of membrane depolarization and the select neuronal systems activated (Pisani et al., 2005). Under the present experimental conditions (ACSF containing Mg2+), the synaptic change was LTD as previously observed in the rodent corticostriatal pathway (Calabresi et al., 1992a,b; Centonze et al., 1999, 2001; Chen et al., 2006). Interestingly, the synaptic plasticity lost with nigrostriatal damage was preserved in lesioned animals with chronic nicotine treatment, that is, LTD was restored. Because synaptic plasticity in basal ganglia motor circuits has been proposed as a neuronal mechanism underlying motor learning and memory (Pisani et al., 2005), these data support the idea that chronic nicotine administration helps maintain/restore normal nigrostriatal function.

In the current study, nicotine was administered before and during the development of nigrostriatal damage. This regimen was selected because our intent was to determine whether the nicotine in tobacco might account, at least in part, for the decreased incidence of Parkinson’s disease with smoking. With the current nicotine-dosing schedule, striatal function may have normalized because nicotine improved function after injury and/or reduced the MPTP-induced nigrostriatal degeneration. This damage was most likely attributable to the neurotoxic effects of MPTP, but may also have occurred because of compensatory increases in dopamine turnover and fractional dopamine release after nigrostriatal damage (Zigmond et al., 1990; Zigmond, 1997; Elsworth et al., 2000; McCallum et al., 2006a). Elevated dopamine levels have been shown to increase production of reactive oxygen species such as peroxides, superoxides, and hydroxyl radicals and dopamine quinones in residual dopamine terminals (Halliwell, 1992; Hastings et al., 1996; Blum et al., 2001). These cytotoxic products may augment the existing nigrostriatal damage (Rosenberg, 1988; Masserano et al., 1996; McLaughlin et al., 1998; Berman and Hastings, 1999).

Activation of these signaling mechanisms may subsequently lead to neuroprotection through a variety of mechanisms including changes in synaptic plasticity. For instance, in the present studies, we observed that chronic nicotine treatment facilitated corticostriatal synaptic plasticity or LTD, a function dependent on nAChR-evoked dopamine release (Partridge et al., 2002). The glutamatergic corticostriatal pathway is the major excitatory input to the basal ganglia (DeLong, 1990; Partridge et al., 2002; Kolomiets et al., 2003). Chronic nicotine treatment may attenuate neuronal damage by reducing glutamatergic excitoxicity in the striatum through an enhanced LTD.

In addition, nicotine may act through receptor-independent mechanisms. These include a direct antioxidant effect possibly through lipid peroxidation and/or by scavenging reactive oxygen species such as superoxides (Soto-Otero et al., 2002; Cormier et al., 2003). Nicotine also appears to compete with NADH to attenuate mitochondrial respiratory control (Cormier et al., 2003), and to reduce mitochondrial swelling and cytochrome c release from mitochondria in the presence of nAChR blockers (Xie et al., 2005). In addition, liver metabolizing enzymes such as cytochrome P450 are induced by nicotine (Tyndale, 2003; Hukkanen et al., 2005).

It is unlikely that nicotine had a direct effect on MPTP biodisposition, because previous studies had shown that nicotine only affected dopaminergic measures in the striatum and not dopaminergic cell number in the substantia nigra (Quik et al., 2005b). As well, the activity of monoamine oxidase, the enzyme responsible for the conversion of MPTP to the active metabolite MPP+, was not different in animals treated with or without nicotine (our unpublished observation).

The primate striatum is anatomically diverse and receives differential dopaminergic inputs from discrete regions in the substantia nigra to result in ventromedial to dorsolateral innervation gradients in the striatum. These gradients are associated with distinct patterns of striatal degeneration with nigrostriatal damage, with more severe dopaminergic losses in dorsolateral compared with ventromedial regions and greater declines in the putamen than caudate in animal models and Parkinson’s disease (Parent et al., 1983; Hirsch et al., 1988; Kemel et al., 1989; Gibb et al., 1990; Graybiel et al., 1990; Fearnley and Lees, 1991; Parent and Lavoie, 1993). The present data also exhibit ventromedial to dorsolateral gradients in MPTP-lesioned animals with a more pronounced neurodegeneration in the putamen than caudate. Moreover, the functional restoration with nicotine treatment parallels these morphological gradients.

The finding that nicotine administration ameliorates function in monkeys with nigrostriatal degeneration raises the question whether it may be of therapeutic benefit. To date, administration of the nicotine patch and/or gum has yielded conflicting results, with some studies reporting small improvements in motor and cognitive deficits and others no effect or a decline (Ishikawa and Miyatake, 1993; Fagerstrom et al., 1994; Ebersbach et al., 1999; Kelton et al., 2000; Vieregge et al., 2001; Lemay et al., 2004). Drawbacks of these studies include the limited number of subjects and very short duration (2–4 weeks). The current experimental design would suggest that longer treatment periods with nicotine are required. The use of nicotine for Parkinson’s disease therapy therefore requires further study.

In summary, the present results show that chronic nicotine treatment tends to normalize lesion-induced overactivity of the nigrostriatal pathway, with an attenuation of the enhanced striatal dopamine turnover and evoked dopamine release. In addition, nicotine treatment preserves synaptic plasticity that is lost with nigrostriatal damage. These results suggest that long-term nicotine administration has potential as a treatment strategy to either protect against or restore dopaminergic terminal function in Parkinson’s disease, a chronic disorder with progressive neurodegeneration.

Footnotes

This work was supported by National Institutes of Health Grants ES012077, NS42091, and NS47162 and the Michael J. Fox Foundation.